Strip wound flex tube or hose has been in existence for over 100 years. Some common uses for strip wound flex tube include flexible electrical conduit, flexible pneumatic hose and flexible hose for use in truck exhaust systems. Throughout the specification, the terms "hose" and "tube" will be used interchangeably.
Strip wound flex hose is sometimes referred to as interlocked "S" hose. Typically, flexible hose is made of a strip of metal having a generally "S" shaped cross section including a first hook portion positioned opposite from a second hook portion. To form a hose body, the strip of metal is wrapped in a helical pattern. As the strip of metal is wrapped in the helical pattern the first and second hook portions of adjacent helical wraps are interlocked to form pivotal interlock seams of the hose body. Typical metals used in manufacturing flexible hose include galvanized steel, aluminized steel, and stainless steel.
A flexible hose segment used in a truck exhaust system is subjected to a very harsh, destructive environment. Hence, flexible hoses in truck exhaust systems are more likely to fail sooner than flexible hoses used in less harsh environments such as electrical or pneumatic applications. Common causes of flexible hose failure in truck exhaust systems include: (1) heat; (2) vibration from the engine and truck; (3) displacement from frame twists and engine motor mount flexing; (4) corrosion and carbon contamination; and (5) wear.
Engine heat causes thermal expansion of the flexible hosing incorporated within an exhaust system. A new piece of flexible tubing has sufficient flexibility to inhibit thermal stresses on the exhaust system parts. Unfortunately, heat causes the flexible tube to grow progressively more rigid with age. Consequently, older flexible tubing is subject to breakage due to thermal expansion/contraction.
Vibrations causing damage to flexible hoses in vehicle exhaust systems are commonly caused by engine vibrations and pressure pulsations in the exhaust gas flow. Such vehicular vibrations cause slight relative movement between the individual wraps of the flexible hose. The relative movement between adjacent convolutions of the hose causes the convolutions to rub against one another and abrade the flexible hosing.
Displacement of exhaust system tubing is produced primarily when a vehicle is shifting gears, or when the vehicle frame is twisted by impacts to the frame such as road bumps. Twisting of the vehicle frame produces bending moments on the exhaust system tubing and muffler that can cause breakage. Changes in torque associated with shifting gears can also cause vehicle engines to displace in their mounts thereby causing exhaust tubing connected to the engines to be displaced. New flexible tubing can readily absorb the movement associated with frame twist or engine torque motion. However, such movement can cause breakage of older, less flexible tubes.
Corrosion and carbon contamination also are responsible for flexible tubing failure. Depending on the choice of metal, exterior corrosion of a piece of flexible tubing is typically caused by rain, snow, and road salt. As the outside of the flexible tubing corrodes, the individual convolutions or wrapped segments become rigid with respect to one another. Heat can also cause corrosion or scaling of the metal. Carbon produced by diesel engines, along with the products of combustion such as weak acid, can further cause internal deterioration and inflexibility of flexible tubing.
The various factors mentioned above all contribute to flexible tubing failure. Typically, wear failure is caused by a combination of the above factors. For example, often a segment of flexible tube becomes inflexible forcing all motion to be concentrated on one convolution of the flexible tube segment. Frequently, the one convolution is located in the center of the segment. This, in turn, causes excessive motion and forces on the center of the segment which cause the flexible tubing to break at the center. Alternatively, a bending moment can be greatest at an end of a piece of tubing causing the tubing to break adjacent to the clamp.